It is well known in the art of mobile radiotelephones to employ a receiver for receiving a receive frequency signal while a transmitter simultaneously transmits a transmit frequency signal in the other direction, the transmit frequency being separated from the receive frequency by a constant offset known as the duplex spacing.
Although the duplex spacing is nominally a constant, it can be a different constant depending on the frequency band in which the mobile phone is operating. Complications can then arise in constructing mobile phones that operate in more than one frequency band.
U.S. patent application Ser. No. 08/795,930 entitled "Transmit Signal Generation with the Aid of Receiver" (Dolman) describes the use of the second local oscillator of the receiver as a reference frequency against which a transmit frequency is controlled relative to a receive frequency to achieve either a first or a second duplex spacing. The Dolman application is hereby incorporated by reference.
It is also known in the prior art, when packaging two synthesizer PLL circuits into a common integrated circuit, to synchronize or otherwise relate the reference dividers of the two PLLs so that their phase comparators do not mutually interfere. The Philips UM1005 and 8026 dual synthesizer integrated circuits available on the open market use this technique. These circuits include the use of fractional-N dividers and programmable loop bandwidth, such as described in U.S. Pat. Nos. 5,095,288 and 5,180,993 which are hereby incorporated by reference. Novel ways to employ such synthesizers in dual mode satellite/cellular telephones in order to achieve different tuning step sizes in different frequency bands are described in U.S. Pat. Nos. 5,535,432 and 5,610,559 which are also hereby incorporated by reference.
Continuous advances in electronics allow for smaller mobile phones complying with a variety of national and international protocols. The international mobile phone standard known as GSM in Europe and as PCS 1900 in the USA operates with a transmit/receive duplex spacing of 45 MHz in the European 900 MHz band; 95 MHz in the European 1800 MHz band, and 80 MHz in the U.S. 1900 MHz PCS band. The channel spacing is 200 KHz (13 MHz/65) and the transmitted symbol rate is 13 MHz/48. All timing in this standard is related to a 13 MHz clock, as is well known. The U.S. IS 136 system known as DAMPS operates with a 45 MHz duplex spacing in the US 800 MHz cellular band, and with an 80.4 MHz duplex spacing in the U.S. 1900 MHz PCS band, with a tuning step size of 30 KHz and a transmitted symbol rate of 24.3 Kilosymbols/sec. In IS 136, as is well known, the tuning step sizes and symbol rates and internal timing are all derivable from a 19.44 MHz clock. Yet another U.S. standard known as IS95 uses Code Division Multiple Access at a transmitted chip rate of 1228.8 MHz, with a duplex spacing of 45 MHz combined with tuning steps of 30 KHz in the 800 MHz band, alternatively 50 KHz steps combined with 80 MHz duplex spacing in the 1900 MHz band. In IS95, the chip rate and frequency step sizes are not easily derivable from the same crystal oscillator. It may be easily understood that combining two or more of the abovementioned protocols in the same handheld unit is hindered by the variety of tuning step sizes, duplex spacings and symbol rates that must be synthesized. Consequently, there exists a need for an improved radio architecture to facilitate such combination.
As mobile phones are designed with fewer battery cells, the requirements for a suitable VCO (voltage controlled oscillator) in the receiver become more complex. The requirements present significant challenges for a 3 cell dual band phone. The VCO single sideband (SSB) phase noise determines the close in intermodulation performance of the receiver. The ability to meet this requirement is related to the gain (K.sub.v) of the VCO and is more difficult as the gain is increased. The gain has the units of MHz/volt where MHz is the amount of frequency coverage of the VCO and volt is the amount of voltage that is available to move the varactor capacitance. For a fixed tuning range of, for example, 75 MHz, the VCO gain K.sub.v increases with decreasing battery voltage. For a 5-cell phone the tuning voltage range would be 0.5 V to 4.3 V for a K.sub.v of 20. A 3-cell phone has a range of 0.5 to 2.2 V for a K.sub.v of 44. It is much more difficult to meet the SSB phase requirement in a 3-cell phone with one VCO for both bands given this significantly higher K.sub.v.
One way to solve the close in intermodulation problem of a low-voltage dual-band receiver is to have a separate receive VCO for each band in a dual band phone. However, this solution requires 2 VCOs for a dual band receiver, and therefore is expensive and not space efficient. A second way would be to have a switched tank circuit for the receiver VCO. However, this solution is unacceptable because it draws current, which significantly decreases standby time in the band where the switch draws current.